Acquired immunodeficiency syndrome (AIDS) is a degenerative disease of the immune system and central nervous system (CNS) resulting from infection of humans by HIV virus. AIDS is responsible for a rapidly growing fatality rate in the world population. At present, no cure has been found, and clinically approved drugs are limited in number. These drugs include nucleoside reverse transcriptase (RT) inhibitors such as 3′-azido-3′-deoxythymidine (AZT, Zidovudine), dideoxyinosine (ddI, Didanosine), dideoxycytidine (ddC, Zalcitabine), 2′,3′-dideoxy-3′-thiacytidine (3TC, Lamivudine), and 2′,3′-didehydro-3′-deoxythymidine (d4T, Stavudine), a non-nucleoside RT inhibitor (Niverapine), and protease inhibitors such as saquinavir (Inverase), ritonavir (Norvir), indinavir (Crixivan), and nelfinavir (Viracept). Nucleoside RT inhibitors generally have similar structures (2′,3′-dideoxynucleosides) and act at an early stage in virus replication to inhibit provirus DNA synthesis (De Clercq, 1995, Journal of Medicinal Chemistry, 38:2491-2517). However, AZT, the recommended initial therapeutic agent, and the other nucleoside analogues have several limitations, including adverse side effects such as bone marrow depression and anemia (Gill et al., 1987, Annals of Internal Medicine, 107:502-505; Richman et al., 1987, New England Journal of Medicine, 317:192-197). Peripheral neuropathy is also a major and common side effect. AZT is rapidly eliminated from the plasma with a half-life of about one hour (Surbone et al., 1988, Annals of Internal Medicine, 108:534-540) and is quickly metabolized in the liver to its corresponding 5′-glucuronide, which is inactive.
Presently, only a small number of antiviral drugs are available for treatment of virus infections. A complication to the development of such drugs is that mutant strains of virus which are resistant to currently available antiviral drugs are developing at an alarming rate. Combinations of new drugs having unique modes of action are urgently needed to replace drugs that have lost their potency against viruses as a result of virus mutations. A further complication to the development of antiviral drugs is that development of viral resistance to available compounds is not the same in different body compartments and fluids. For example, evolution of drug resistance among HIV-1 clinical isolates is often discordant in blood and semen of HIV-1 positive males (Eron et al., 1998, AIDS 12:F181-F189).
Further, currently available drugs useful for antiviral therapy sometimes ineffectively penetrate the genital tract. This is a serious drawback to the use of these drugs to combat viruses which infect the genital tract. If an antiviral drug promotes development of resistance in the genital tract and the virus is commonly transmitted from this body site, the drug will rapidly become ineffective for treatment of the virus infection in the population at risk for transmission. Hence, drug-resistant mutants of certain viruses can be rapidly spread by sexual contact in the human population. It is known that viruses such as HIV, hepatitis B, hepatitis C, herpes simplex virus, cytomegalovirus, papilloma viruses, and many others are transmitted via sexual contact by both males and females. Thus, therapeutic drugs that fully suppress virus infections in the genital tract are a high public health priority.
Another limitation of presently available antiviral drugs is that rapid emergence of drug resistant mutant virus can lead to decreased sensitivity to the drug within a patient or within a patient population (Larder et al., 1989, Science, 243:1731-1734). Thus, the beneficial effects of drugs such as AZT are limited in duration.
The anti-HIV chemotherapy era which started a decade ago has recently made significant progress toward better control of HIV-1 infection by the introduction of protease inhibitors and the use of combinations of nucleoside and non-nucleoside RT inhibitors with protease inhibitors. Monotherapy (e.g. administration of a single drug) using a nucleoside or non-nucleoside RT inhibitor or a protease inhibitor is no longer a recommended form of therapy for treatment of a patient with a virus infection such as HIV-1 infection. Although combinations of AZT, 3TC, and a protease inhibitor have reduced virus load in the plasma of patients to below detectable levels (i.e. fewer than 200 copies of viral RNA per milliliter of plasma) with a concomitant increase in CD4+ cell count, some drug combinations have been associated with increased toxicity in a person receiving multiple drug therapies. Also, although reduction in virus burden in the plasma of patients to non-detectable levels achieved using some drug combinations is impressive, drug resistance is an escalating problem due to both use and misuse of drug therapy (De Clercq, 1995, Journal of Medicinal Chemistry, 38:2491-2517; Bartlett, 1996, Infectious Diseases in Clinical Practice, 5:172-179) and evolution of resistant mutants in blood and seminal fluids (Eron et al., 1998, AIDS, 12:F181-F189).
The pathogenic events in HIV disease have recently been reviewed by Fauci (1996, Nature {New Biology}, 384:529-534). The current understanding is that entry of HIV into cells varies with the virus strain and cell type. Primary infection of humans is associated with macrophage tropic (M-tropic) virus that utilize the CD4 receptor and a beta-chemokine co-receptor (CCR5) for entry into macrophages. As HIV infection progresses, the initial M-tropic viruses are usually replaced by T-tropic viruses that enter T-lymphocytes via the CD4 receptor and co-receptor CXCR4 (fusin). The viral determinant of cellular tropism maps to the gp120 subunit of HIV-1 Env protein, particularly the 3rd variable region or V3 loop of gp120. Upon entry into these cells, HIV probably infects dendretic cells, which then carry the virus to CD4+ cells in the lymphoid organs. Infection is then established in the lymphoid organs and a burst of infectious virus seeds itself throughout the body, including the CNS, brain, and lymphoid tissues and sexual organs (e.g. testes). Current drugs used in therapies for HIV infection and AIDS noted above have a limited capacity and half-life for absorption from the stomach to the blood, accumulation into lymphoid organs, crossing the blood-brain barrier into the CNS, or entering the sexual organs (e.g. testes) to attack sanctuaries for HIV replication.
Synthetic phosphocholine lipid (PC lipid) analogues such as, for example, 1-decanamido-2-decyloxypropyl-3-phosphocholine (INK-11) have demonstrated a low incidence of unwanted side effects in mice such as reduction of bone marrow precursor cells and have exhibited high differential selectivity (i.e. the ratio of TC50 for cytotoxicity to EC50 for antiviral activity, DS=1342 for INK-11) in human leukocytes in cultured cells. At a dosage of 50 milligrams per kilogram of body weight per day for 21 days, INK-11 inhibited Friend leukemia virus-(FLV-) induced pathogenesis by 42% in infected mice, as indicated by significant activity against splenomegaly. The observation that use of INK-11 resulted in only moderate suppression against RT activity compared with AZT alone (42% vs 98%, respectively) suggests that INK-11 induces production of defective virus, similar to the effect achieved using other lipid compounds alone (Kucera, et al., 1990, AIDS Research & Human Retroviruses 6:491-501).
Other synthetic phospholipids which do not comprise a phosphocholine moiety (non-PC lipids) have been conjugated with antiviral chemotherapeutic agents. For example, thioether lipid-nucleoside conjugates have exhibited improved antineoplastic activity in tumor-bearing mice (Hong et al., 1990, Journal of Medicinal Chemistry 33:1380-1386). Also, natural phospholipids coupled to AZT or to dideoxynucleosides (ddT, ddC) have proven to be markedly active against HIV by inhibiting viral RT activity (Steim et al., 1990, Biochemical & Biophysical Research Communications 171:451-457; Hostetler et al., 1990, Journal of Biological Chemistry 265:6112-6117; Hostetler et al., 1991, Journal of Biological Chemistry 266:11714-11717). Studies of phospholipid antiviral efficacy have also included chemically conjugating AZT or ddI, through a phosphate-ester bond, to selected synthetic phosphatidic acid lipid analogues (Piantadosi et al., 1991, Journal of Medicinal Chemistry 34:1408-1414). Synthetic phosphate-ester linked lipid-nucleoside conjugates were found to be markedly active against infectious HIV-1 production in both acutely- and persistently-infected cells, and were 5- to 10-fold less cytotoxic compared with AZT alone (Piantadosi et al., 1991, Journal of Medicinal Chemistry 34:1408-1414). Results of preliminary studies indicated that synthetic lipid-AZT conjugates block reactivity of HIV-1-induced gp160/gp120 proteins with specific monoclonal antibodies on the surface of infected and treated cells and on the surface of treated HIV-1 particles, as measured by flow cytometry. These conjugate compounds also caused inhibition of HIV-1-induced cell fusion (Kucera et al., 1992, In: Novel Membrane Interactive Ether Lipids With Anti-Human Immunodeficiency Virus Activity, Aloia et al., eds., Membrane Interactions of HIV, pp. 329-350; Krugner-Higby et al., 1995, AIDS Research & Human Retroviruses 11:705-712). However, these phosphate ester-linked lipid-AZT conjugates (non-PC lipid-AZT conjugates) were not very active against AZT-resistant clinical isolates of HIV-1. Moreover, after intracellular metabolism of the conjugate with resulting release of AZT-monophosphate, the lipid moiety exhibited only moderate to non-detectable antiviral activity (Piantadosi et al., 1991, Journal of Medicinal Chemistry 34:1408-1414).
As with the antiviral agents, the development of anticancer agents for treating cancer effectively has also been problematic. Barriers such as cellular mechanisms of anticancer drug resistance, overcoming the blood-brain barrier to provide adequate delivery of drug to the brain and CNS, inadequate uptake of drug by lymphoid and hematopoietic tissues, toxicity, achieving oral bioavailability, overcoming short drug half-life, and preventing extracellular metabolism of the anticancer agent are faced by the skilled artisan. In order to improve bioavailability to CNS and brain tissue, nucleoside analogues have been encapsulated in liposomes or used with modifying agents to disrupt the blood-brain barrier (Braekman, et al., 1997, Proc. Amer. Soc. for Clinical Oncology, Abstract #810). Implantable devices have been used to provide more sustained drug delivery to increase the pharmacokinetics of anticancer agents (Del Pan, et al., 1997, Proc. Amer. Soc. for Clinical Oncology, Abstract #1384). Additionally, attempts to improve the efficacy of nucleoside analogues in cancer therapy have included the use of multidrug combinations and high-dose nucleoside analogue therapy (Capizzi, 1996, Investigational New Drugs 14:249-256). None of these methods have adequately overcome the problems discussed above with regard to anticancer agents.
Another attempt to circumvent the problems associated with conventional nucleoside analogue cancer therapy has been the conjugation of these molecules to phospholipids. Thus far, the conjugation of nucleoside analogues to phospholipid molecules has focused on ara-C and a limited number of diacyl, alkylacyl and thioether phospholipids (Hong, 1990, Cancer Res. 50:4401-4406). Although these conjugates have shown efficacy in the treatment of hematologic malignancies, these drugs must be administered intraperitoneally or intravenously and do not overcome the problems discussed above regarding anticancer agents. These conjugates are degraded by phospholipase A and phospholipase B extracellularly and do not provide the option of oral administration.
Despite the promising attributes of compounds such as PC lipids, and non-PC lipid-nucleoside analogue conjugates, currently available antiviral and anticancer agents such as nucleoside analogues and anti-HIV nucleoside drugs have severe inherent limitations. Although such drugs are capable of delaying the onset of symptoms of virus infection and extending survival time for patients, new compounds having the attributes of increased tolerability, potency, and selectivity against specific viruses, differential mechanisms of action, ability to cross the blood-brain barrier, and freedom from myelosuppressive side effects are urgently needed for improved treatment of virus infections. Also, new antiviral and anticancer compounds are needed which more effectively combat cancers or target multiple aspects of the virus life cycle, which facilitate delivery of an anticancer agent to cells and tissues not normally accessible to anticancer agents (e.g. CNS and lymphoid tissues), which combine lipophilic (e.g. phospholipid) and antiretroviral or anticancer agents within the same molecule (e.g. conjugate compounds) in order to yield a drug with a more sustained antiviral or anticancer effect, which decrease the rate of emergence of drug-resistant virus strains, and which inhibit virus replication in a wider range of cellular or tissue reservoirs of virus infection. The present invention satisfies these needs.